MakeParticle and elasticity plugin refactoring.

- Removed the explicit dependency on a plugin name.
- Creation of particles from user-defined vertices (if a plugin is present).
- Add skin support for an irregular mesh
- Fixes #642 and #674.

PiperOrigin-RevId: 561313771
Change-Id: I450f91f0d3b578cc18d97ed7180a0a8944b7dd41

src/user/user_composite.h

@@ -106,9 +106,13 @@ class mjCComposite {
   // currently used only for cable
   std::string initial;            // root boundary type
   std::vector<float> uservert;    // user-specified vertex positions
+  std::string userface;           // connectivity
   mjtNum size[3];                 // rope size (meaning depends on the shape)
   mjtCompShape curve[3];          // geometric shape
 
+  // body names used in the skin
+  std::vector<std::string> username;
+
   // plugin support
   bool is_plugin;
   std::string plugin_name;